1. Field of the Invention
The present invention relates to a voltage stabilizing circuit, and more particularly to an output voltage stabilizing circuit of a switching power supply circuit used for, for example an electronic exchange system.
Generally, in various electronic circuits, when a power supply voltage fluctuates, the operating point of a transistor or integrated circuit (CI) is changed so that the originally intended performance cannot be obtained. To prevent this, the output voltage of the power supply circuit is detected by a voltage detecting circuit, and when the detected voltage deviates from a predetermined reference voltage, the power supply circuit is controlled based on the deviation so as to output a stable power supply voltage. The present invention relates to a voltage stabilizing circuit for the above case as an example.
2. Description of a Prior Art
In a conventional voltage stabilizing circuit, a voltage detecting circuit is realized by a mirror circuit including two transistors arranged symmetrically. With the mirror circuit, however, the relation between the output voltage of the voltage stabilizing circuit and the detected voltage detected by the voltage detecting circuit depends on a base-emitter voltage of one of the two transistors. The base-emitter voltage of the transistor has a temperature characteristic such that the base-emitter voltage fluctuates depending on the temperature. Therefore, in the conventional voltage stabilizing circuit, there is a problem in that the accuracy of the detected output voltage is too low because of the fluctuation of the base-emitter voltage which may be caused when the temperature of the environment for the voltage stabilizing circuit is changed or when an abnormal accident occurs at the output side of the power supply circuit to increase the temperature.
It should be noted that such a problem is generated in not only the case of the switching power supply circuit explained above as an example of the application of the voltage stabilizing circuit, but in a series regulator that keeps the output voltage constant by controlling a transistor or a variable resistor, or in a voltage detecting circuit used in various other electronic devices.